The present invention relates generally to a method of fabrication of superconducting articles and more particularly to the fabrication of large bulk high temperature superconducting (HTS) articles.
In 1992 a scientific paper was published suggesting that large high temperature superconductors of YBCO material might interact with the local gravity field when spun and exposed to certain electromagnetic fields. The effect required at least a 12 inch diameter annular disk in the experimental apparatus. Several such disks were created for this experiment in Moscow, Russia. But no details of the methods used to create the large disks were revealed. Smaller pieces of this material have been fabricated for years. But to the best of our knowledge, the present large disk is the first of this size. No patents or scientific literature have been found describing the present method of producing such large superconducting articles. We believe that the process used herein is the first to successfully fabricate such a disk.
The objective was to create large pieces of HTS to create a large annular disk of the material. Problems encountered and overcome included problems of creating the green (unfired) piece which did not break apart prior to firing. This material is fragile and does not hold together well, unless it is processed properly. Small pieces are relatively easy to fabricate; however, the internal stresses in a large piece have always resulted in cracks and structural failures in the green disk.
Once the green disk is successfully fabricated, it must be fired to create the final item. Firing has also been a problem with large pieces, since the thermal stresses caused by large temperature changes and gradients across the piece resulted in cracks and structural defects.
The present invention solves these problems with the result that large pieces of HTS can be successfully fabricated. The initial fabrication applies to a 12 inch diameter annular disk; however, the method can be extended to other shapes and sizes. Note that many patents exist which claim methods for formulating the chemical constituents of such ceramic superconductors. The methods described herein focuses on the physical conditioning of the materials rather than the chemical. The methods described herein can be used with most (if not all) of the chemical formulations used to manufacture HTS bulk items.
Accordingly it is an object of the present invention to provide a method of fabrication of large bulk high temperature superconducting articles.
In accordance with the present invention, the method of fabrication of large bulk high temperature superconducting articles includes the processes of material preparation, compression and heat treatment.
Material preparation includes the step of mixing superconducting powders such as YBCO or BISCCO into a homogeneous mixture. Particle sizes are chosen which are in a ratio of 1 to 3 to 10. That is, particle sizes of 50 microns, 150 microns and 500 microns in diameter.
In the compression step, the homogeneous mixture is then placed in a die which is then placed in a press and compressed at a predetermined pressure for a predetermined time.
In the heat treatment step, the green piece is removed from the die and then placed in a furnace and heated to sintering temperature and then allowed to soak at predetermined temperatures and then (while in the furnace) is oxygenated for a predetermined time before it is removed.